Kinetics of Hydrolysis of Phenyl Acetates Catalyzed by the Zinc(II) Complex of 1,5,9-Triazacyclododecane. Evidence for Attack of Water or Hydroxide Ion at the Coordinated Ester

Rates for hydrolysis of substituted phenyl acetates were measured in the presence of the Zn(II) complex of 1,5,9-triazacyclododecane. The kinetic data are incompatible with the mechanism in which the water and hydroxide ion coordinated to the metal center makes a nucleophilic attack at the ester linkage. Instead, the results support the mechanism in which the ester is first complexed to the metal center and then water or hydroxide ion makes a nucleophilic attack at the complexed ester. The kinetic data further indicate that the electrostatic interaction between the cationic metal center and the anionic hydroxide ion facilitates the attack of hydroxide ion at the complexed ester. In addition, the C-O bond of the ester is not cleaved significantly in the rate-determining transition state for attack by both water and hydroxide ion.